If you're working on a Linux console,
loadkeys is indeed the right command. The argument is the path to a keymap file from the current directory or from
/usr/share/keymaps; if you don't include a directory component then
loadkeys will also search subdirectories of
/usr/share/keymaps recursively. Furthermore you can leave out
.kmap at the end and supported compression suffixes. For example (assuming BackTrack includes the same keymaps as Debian/Ubuntu)
loadkeys fr will give you a French layout,
loadkeys be-latin1 will give you a Belgian layout, etc. There are several variations with additional non-ascii characters.
If you're working in an X environment, you need to use XKB or
xmodmap or a GUI tool.
xmodmap is the simplest of the two; the keymaps are in
/usr/share/xmodmap (on Debian/Ubuntu, I haven't checked that they're in the same place on BackTrack). For example
xmodmap /usr/share/xmodmap/xmodmap.fr will give you a French layout.
An alternative is to log into the machine via ssh. Then you would have your favorite keymap anyway. (Of course you'd have to type something in the machine first to enable an ssh daemon.)